Slow motion video of
firing a .50 calibre Blackpowder pistol

As can be seen in this
short clip, the effects of recoil on the shooter are quite considerable, notice
the way his head moves back and forth in such a short time (about a fifth of a
second). By using two hands to support the gun, the extra weight of the left
hand helps to reduce the amount of muzzle jump and so the gun can be kept under
better control than if just one hand were used.